Thursday, October 28, 2010

Seafloor Still About 90 Percent Unknown, Experts Say

"The spatial resolution is down to sub-meter [less than three feet] accuracy. In shallow waters on the [continental] shelf we can measure five-centimeter [two-inch] bed forms. But we can't do [these things] all over the world."

Survey ships drop a "beam" below them to map narrow swaths of the seafloor. Mapping all the oceans this way might take a thousand years of ship time and cost tens of U.S. billions of dollars.

"Technology has improved dramatically in the last ten years, but the deep oceans are just so vast," Gardner said. "Something else has to be done."

The Naval Oceanographic Office (NAVOCEANO) operates seven survey ships from the John C. Stennis Space Center in south Mississippi.

NAVOCEANO has also taken to the air in an attempt to more efficiently map the shallow waters along coastlines.

The Compact Hydrographic Airborne Rapid Total Survey (CHARTS) system uses laser technology and digital imaging to quickly map the ocean floor from the air.

"The system uses two lasers," explained Barbara Reed, NAVOCEANO's hydrography department director. "One laser penetrates to the bottom, while another is reflected off the surface. The difference between them is the water depth."

CHARTS cannot penetrate murky waters, such as those near some river mouths, but it has proven effective over large stretches of coastline.

"Along many coastlines we can chart at 125 knots versus 12 knots in a ship. It's significantly more efficient."

Ocean Surface Reflects Seafloor Topography

At the National Oceanic and Atmospheric Administration, Walter Smith works from even higher in the sky with satellite altimetry images able to create a wide, general view of ocean-floor topography.

The system uses an indirect technique, mapping the shape of the ocean surface to indicate the mountains, trenches, and other features that lie below.

"If a mountain is on the seafloor, [its] gravity field pulls extra water to that point and causes the [surface] water to bulge," Smith explains. Conversely [the ocean surface] is sunken toward the Earth over a trench."

The satellite images present a "smoothed out" picture of ocean-floor topography rather than a detailed picture.

"People say, Gee, if isn't good enough to pinpoint a reef or to navigate, why bother?" Smith said. "Most people don't realize that almost all of the ocean is unmapped by traditional means."

Satellite data can be helpful to surveying ships. "Satellite altimetry can be a good indicator of where you need to survey," Best said. "It's rough, it's not the fidelity needed for navigation, but in helping to prioritize where you need to survey, it can be of great use."

A diverse group of scientists could also benefit from an update of the satellite data, which is 10 to 20 years old.

The images paint what Smith calls "a pretty clear picture" of the Mid-Ocean Ridgea view quite different from what might have been assumed from scattered, detailed measurements at points along the system of underwater mountains that snakes its way between the continents.

"The big picture provides enough detail to say if you don't understand a certain aspect of seafloor spreading or plate tectonics, you might want to work here," Smith said.

Tsunami-Preparation Efforts

Such maps could also be critical for tsunami-preparation efforts.

"No matter how deep the ocean is, [a tsunami] still feels the bottom, and its path is still influenced by what's on the bottom," Smith said.

Thus, understanding the location of trenches, seamounts, and other features is essential to calculations of how a tsunami will move and where and in what force it will come ashore.

Other studies that could benefit from mapping include marine animal habitat and ocean mixing rates, which are essential to absorption of greenhouse gases. All are dependent on more detailed knowledge of the "other" 70 percent of Earth's surface.

"The uninitiated think that we've mapped the whole world," said the University of New Hampshire's Gardner. "But we really do know more about the back side of the moon than we do the ocean floor."